Friction separator paper pick mechanisms are commonly used in printers and copiers, for example, to feed a single sheet of paper into transport rolls, which forward the single sheet to a process station of the printer or copier. A typical friction separator paper pick mechanism includes a spring loaded paper-lift plate, a high friction pick roll, and a spring loaded separator pad.
The separator pad is formed of a material having a coefficient of friction with paper greater than the coefficient of friction between adjacent sheets of paper but smaller than the coefficient of friction between the pick roll and a sheet of paper. This relationship of the coefficients of friction insures that the pad will not prevent advancement of the sheet by the pick roll but will separate any sheet beneath the uppermost sheet. The geometry of the mechanism has both the paper-lift plate and the separator pad contact the high friction pick roll with the uppermost sheet in the paper-lift plate contacting the high friction pick roll prior to the sheet being fed between the separator pad and the high friction pick roll.
When actuated to advance a sheet of paper from a stack, the paper-lift plate moves to a position in which the uppermost sheet in the stack is engaged by the intermittently driven pick roll so that the uppermost sheet is fed into a nip formed between the pick roll and the separator pad. If only a single sheet is picked by the pick roll, the fed sheet will pass through the nip formed between the pick roll and the separator pad into the printer or copier because of the high coefficient of friction between the pick roll and the sheet in comparison with the coefficient of friction between the separator pad and the sheet. If two or more sheets are picked by the pick roll as occurs with many high friction media, the purpose of the separator pad is to restrain all but the uppermost sheet in the stack from being advanced during a specific cycle of operation.
After the sheet is fed to the transport rolls for advancement into the printer or copier, it is critical to minimize drag on the fed sheet in an edge aligned printer or copier. This is because one of the sheet's side edges rides along guide means as the sheet is transported by relatively small rollers and relatively low nip forces.
In an edge aligned system, extraneous drag on the sheet can cause skew of print and other imaging degradation. In severe cases, the extraneous drag on the sheet can cause the sheet to slip in the transport rolls whereby the sheet jams in its feed path through the printer or copier.
Since the drag must be minimized in an edge aligned system, the spring load between the sheet and the pick roll is normally removed after the sheet is picked from the stack by the pick roll to open the nip. This is accomplished by either moving the paper-lift plate away from the pick roll or raising the pick roll away from the stack of sheets. The spring load between the separator pad and the pick roll also is removed to prevent the unwanted drag.
This opening of the two nips is the principal contributor to feeding more than one sheet during a cycle of operation in this type of mechanism. This is because opening of the two nips enables one or more of the underlying sheets in the stack to be dragged into the printer along with the uppermost sheet unless the motion of the underlying sheets is retarded in some manner.
Retarding of the motion of the underlying sheets is usually accomplished by rotating an arm having sharp steps, which catch the underlying sheets, into the sheet feed path by the same mechanism, which drops the separator pad from engagement with the pick roll. In this arrangement, the timing, geometry, and tolerances are very critical since the retarding means must be disposed to catch the underlying sheets as the nip is opened or multiple sheets will be fed. Even when the retarding means is disposed in its proper position, media of high friction and low weight in particular still tend to be dragged into the printer or copier by the fed sheet through "jumping" over the retarding means.
This problem is averted in a center driven system by having the spring loaded paper-lift plate and the spring loaded separator pad remain in contact with the high friction pick roll throughout the feeding of the entire stack. This is possible because it is not necessary to open the nips after feeding of each sheet since center driven sheet feeders for printers and copiers have much larger transport rolls, much higher nip forces, and no reference edge with which the sheet must be aligned. Of course, this is a more costly system in comparison with the edge aligned system having the relatively smaller rollers.
The much larger transport rolls of the center driven system can exert a sufficient force to pull each sheet from the nips without having to open either of the two nips. The pick roll is usually driven through a one-way clutch to aid the transport rolls in moving the sheet.
Since the two nips are not opened and closed for each fed sheet in the center driven system, the timing, geometry, and tolerances are not as critical in terms of feed reliability as in the edge aligned system. This is because the sheets are always tightly held in the nips so that there is less chance of the underlying sheets being dragged into the printer or copier along with the uppermost sheet. However, as previously mentioned, the center driven system requires a higher cost for its parts in comparison with the edge aligned system, and it also requires more power to operate.